1. Field of the Invention
This invention relates to a mold for molding when preparing a substrate for information recording medium according to the cast molding method and to a method for preparation thereof.
2. Related Background Art
In the prior art, a substrate for information recording medium has an uneven pattern such as grooves for tracking, pits for information, etc. on the information recording surface thereof. As the method for forming this uneven pattern on the substrate, there have been known (1) the method in which a track groove of the stamper type is transferred by the injection method or the hot press method when the substrate comprises a thermoplastic resin, or (2) the method according to the so-called 2P method, in which a photocurable resin composition is applied on a transparent resin plate comprising a thermosetting resin or a thermoplastic resin, then an energy source such as UV-rays are imparted from the side of the transparent resin plate with the stamper mold adhered thereon to cure the above photocurable resin composition, and the replica of the stamper mold is transferred onto the transparent resin plate.
However, the thermoplastic resin substrate obtained according to the injection method or the hot press method of the first method has the problem that warping or optical anisotropy of the substrate after molding may be caused due to residual stress or orientation of molecules due to thermal history during molding.
On the other hand, the substrate obtained according to the 2P method of the second method will frequently generate the problem of deterioration of recording characteristics, because the polymerization initiator or monomer to be used in photocuring is liable to remain within the resin after photocuring, whereby they have influences on the optical recording layer of the recording medium.
As the method for preparing a substrate for information recording medium is free from such problems, there has been known (3) the method in which during cast molding of a prepolymer containing a monomer of the resin or a solvent, cast molding is effected by use of a mold for casting having track grooves formed previously on one side or both sides thereof. The substrate obtained according to this method has substantially no pressure applied during molding as compared with the methods of (1) and (2). Also, thermoplastic resins, thermosetting resins and photocurable resins can be utilized as the resin for molding, and therefore a resin having no deleterious influence on the recording layer can be chosen.
In the third cast molding method (3), as the mold having an uneven pattern corresponding to the uneven preformat such as grooves for tracking, pits for information, etc., there has been known glass plates subjected to patterning of such unevennesses by photolithographic steps, or to patterning by laser cut, or glass plates, metal plates having unevenness formed thereon by direct cuts with a diamond needle having a sharp end, etc.
However, as a problem in cast molding, there may be low productivity because of very long treatment time at one time during production of the substrate. Accordingly, for the purpose of improving productivity, as a mold to be used in the above cast molding, a mold for giving a plural number of units has been demanded. However, as described above, since minute working is required to form unevenness on the mold for casting, in the case of manufacturing a mold for producing a plural number of units, the production cost will become disadvantageously high. Also, when failures such as defects, etc. occur in one pattern of a plural number of pattern surfaces, there is the problem that the mold itself is no longer usable.